A. Field of the Invention
The invention relates to a method and an apparatus for checking documents of value, such as e.g. bank notes, checks, cards, tickets, coupons.
B. Related Art
From the prior art it is known to provide documents of value with security elements, such as security strips or also security threads, which contain magnetic material. The magnetic material here can be applied onto the security element either continuously or only in certain areas, for example in the form of a coding. For the magnetic coding of a security element there is used for example a certain sequence of magnetic and non-magnetic areas, which is characteristic of the type of the document of value to be secured. Furthermore, it is known to use different magnetic materials for a magnetic coding, for example with different coercive field strengths. In the hitherto known magnetic codings there are used for example two differently coercive magnetic materials, from which there are formed two types of magnetic areas which can be arranged side by side or one above the other.
Further, it is known to automatically check bank notes having security threads, which have a magnetic coding of differently coercive materials. Here, the bank notes are transported parallel to the course of the security element, and one after the other first go through a strong magnetic field parallel to the transport direction, which magnetizes both the high-coercive and also the low-coercive magnetic areas along the transport direction. The remaining magnetization is checked by means of an inductive magnetic head which is sensitive exclusively parallel to the transport direction. Then the bank notes go through a weaker magnetic field perpendicular to the transport direction, which aligns only the low-coercive magnetic areas perpendicular to the transport direction, while the high-coercive magnetic areas remain magnetized in the transport direction. Anew, the remaining magnetization is checked by means of an inductive magnetic head which is sensitive exclusively parallel to the transport direction. With the first inductive magnetic head here the high- and the low-coercive magnetic areas are detected and with the second inductive magnetic head only the high-coercive magnetic areas are detected. If the security element, however, also contains combined magnetic areas, which contain the two differently coercive magnetic materials, so that the differently coercive magnetic materials at the same time reach the detection range of the magnetic detector, an overlay of the magnetic signals of the differently coercive magnetic materials is detected. The combined magnetic areas here deliver a reduced magnetic signal whose signal swing lies between that of the high-coercive and that of the low-coercive magnetic areas. A disadvantage of this method is that these combined magnetic areas can be distinguished only with difficulties from the high-coercive and from the low-coercive magnetic areas.
The invention is therefore based on the object to carry out the check of the documents of value in such a way that the high-coercive, the low-coercive and the combined magnetic areas respectively can be reliably distinguished from each other.